Domain-Driven Design Deep Dive: Key Learnings and Practical Insights

After diving deep into Domain-Driven Design for the new project, here are the key learnings and insights that finally made DDD click for me.

Starting a new project that explicitly uses Domain-Driven Design forced me to really understand what DDD is about beyond just fancy terminology. After hours of reading, experimenting, and applying DDD principles, here are my key takeaways.

Why DDD Matters (My "Aha" Moment)

The real value of DDD isn't in the patterns themselves—it's in the thinking process. It forces you to:

  • Actually talk to domain experts (not just assume you know the business)
  • Model the problem before jumping into implementation
  • Keep business logic separate from technical concerns
  • Design systems that can evolve with business needs

This was eye-opening. I've built plenty of CRUD applications, but DDD made me realize how much business complexity gets lost in translation.

Core Concepts That Finally Made Sense

Ubiquitous Language

This isn't just "use the same words." It's about discovering the exact language the business uses and making sure it's reflected everywhere—code, database, docs, conversations.

Key learning: If you find yourself translating business terms into "developer-friendly" names, you're probably doing it wrong.

// Instead of this generic approach
class UserAccount {
    public function calculateFee() { ... }
}

// Use the actual business language
class PatientAccount {
    public function calculateTreatmentFee() { ... }
}

Bounded Contexts

This was the hardest concept to grasp initially. A bounded context isn't just a microservice—it's a semantic boundary where a particular model applies.

Key insight: The same entity (like "User") can mean completely different things in different contexts:

  • In Billing Context: User = Customer with payment methods and invoices
  • In Medical Context: User = Patient with medical history and treatments
  • In Auth Context: User = Identity with credentials and permissions

Each context gets its own models, even if they represent the "same" thing.

Aggregates

Think of aggregates as consistency boundaries. Everything inside an aggregate must always be in a valid state, and you can only modify an aggregate through its root.

Major learning: Don't make aggregates too big. Keep them as small as possible while maintaining business invariants.

class Order // Aggregate Root
{
    private Collection $orderLines;
    private OrderStatus $status;
    
    public function addOrderLine(Product $product, int $quantity): void
    {
        // Business rule: Can't add lines to shipped orders
        if ($this->status->isShipped()) {
            throw new OrderAlreadyShippedException();
        }
        
        // Keep aggregate consistent
        $this->orderLines->add(new OrderLine($product, $quantity));
        $this->recalculateTotal();
    }
}

Tactical Patterns in Laravel

Entities vs Value Objects

Entities have identity and lifecycle:

class Patient extends Model // Entity - has ID, changes over time
{
    public function updateMedicalHistory(MedicalRecord $record): void
    {
        // Entity behavior
    }
}

Value Objects are immutable and represent concepts:

class Money // Value Object - immutable, no identity
{
    public function __construct(
        public readonly int $amount,
        public readonly Currency $currency
    ) {}
    
    public function add(Money $other): Money
    {
        if (!$this->currency->equals($other->currency)) {
            throw new CurrencyMismatchException();
        }
        
        return new Money(
            $this->amount + $other->amount, 
            $this->currency
        );
    }
}

Domain Events

Game changer for decoupling. When something important happens in your domain, broadcast it:

class Order
{
    public function ship(): void
    {
        $this->status = OrderStatus::SHIPPED;
        
        // Domain event - other parts of the system can react
        event(new OrderWasShipped($this));
    }
}

// Elsewhere in the system
class SendShippingNotificationHandler
{
    public function handle(OrderWasShipped $event): void
    {
        // Send email, update inventory, etc.
    }
}

Repositories

Keep your domain models ignorant of persistence details:

interface PatientRepositoryInterface
{
    public function findByMedicalId(MedicalId $id): ?Patient;
    public function save(Patient $patient): void;
    public function findActivePatients(): Collection;
}

// Laravel implementation
class EloquentPatientRepository implements PatientRepositoryInterface
{
    public function findByMedicalId(MedicalId $id): ?Patient
    {
        $data = DB::table('patients')
            ->where('medical_id', $id->value())
            ->first();
            
        return $data ? Patient::fromArray($data) : null;
    }
}

Strategic Design Insights

Context Mapping

Big realization: Different bounded contexts need to communicate, but they shouldn't share models. Use context mapping patterns:

  • Anti-Corruption Layer: Protect your domain from external systems
  • Published Language: Define a shared format for communication
  • Customer/Supplier: Clear upstream/downstream relationships
// Anti-Corruption Layer for external payment system
class PaymentServiceAdapter
{
    public function processPayment(Order $order): PaymentResult
    {
        // Translate our domain model to external API format
        $externalRequest = $this->translateToExternalFormat($order);
        
        $response = $this->externalPaymentService->charge($externalRequest);
        
        // Translate back to our domain concepts
        return $this->translateToDomainResult($response);
    }
}

Laravel-Specific Implementation Patterns

Service Providers for Domain Services

class MedicalContextServiceProvider extends ServiceProvider
{
    public function register(): void
    {
        $this->app->bind(
            PatientRepositoryInterface::class,
            EloquentPatientRepository::class
        );
        
        $this->app->bind(
            TreatmentPlanService::class,
            function ($app) {
                return new TreatmentPlanService(
                    $app[PatientRepositoryInterface::class],
                    $app[DoctorRepositoryInterface::class]
                );
            }
        );
    }
}

Directory Structure That Actually Works

app/
├── Domain/
│   ├── Medical/
│   │   ├── Entities/
│   │   ├── ValueObjects/
│   │   ├── Events/
│   │   ├── Services/
│   │   └── Repositories/
│   └── Billing/
│       ├── Entities/
│       └── ...
├── Infrastructure/
│   ├── Persistence/
│   ├── External/
│   └── ...
└── Application/
    ├── UseCases/
    ├── DTOs/
    └── ...

Key Lessons Learned

What Worked

  1. Start with the domain model first - Don't think about database tables or API endpoints initially
  2. Talk to domain experts regularly - The domain model should make sense to them
  3. Keep aggregates small - Big aggregates are hard to maintain and scale
  4. Use domain events liberally - They're perfect for decoupling and handling side effects
  5. Don't DDD everything - Some parts of your app are just CRUD, and that's fine

What Didn't Work

  1. Over-engineering simple features - Not everything needs the full DDD treatment
  2. Making aggregates too big - Led to performance and concurrency issues
  3. Ignoring Laravel conventions entirely - DDD doesn't mean abandoning what works in Laravel
  4. Perfect domain models from day one - Domain understanding evolves, so should your models

Common Pitfalls I Hit

  1. Anemic Domain Models: Just getters/setters with no business logic
  2. Domain Services for Everything: Should be entities/value objects doing the work
  3. Sharing Entities Across Contexts: Each context needs its own models
  4. Forgetting About Performance: Domain models still need to be efficient

Practical Tips for Laravel Projects

Testing Domain Logic

// Test domain behavior, not Laravel features
class OrderTest extends TestCase
{
    /** @test */
    public function cannot_add_items_to_shipped_order(): void
    {
        $order = Order::create(/* ... */);
        $order->ship();
        
        $this->expectException(OrderAlreadyShippedException::class);
        
        $order->addOrderLine(
            Product::create('Widget'), 
            quantity: 5
        );
    }
}

Form Requests as Anti-Corruption Layers

class CreatePatientRequest extends FormRequest
{
    public function toDomainCommand(): CreatePatientCommand
    {
        return new CreatePatientCommand(
            medicalId: new MedicalId($this->medical_id),
            name: new PatientName($this->name),
            dateOfBirth: new DateOfBirth($this->date_of_birth)
        );
    }
}

Resources That Actually Helped

  • "Implementing Domain-Driven Design" by Vaughn Vernon - More practical than Evans' original book
  • "Domain Modeling Made Functional" by Scott Wlaschin - Great for understanding the concepts
  • Laravel community discussions - Seeing how others adapt DDD to Laravel

Final Thoughts

DDD isn't a silver bullet, and it's definitely overkill for simple CRUD applications. But for complex domains with intricate business rules, it's incredibly valuable.

The real value isn't in following the patterns perfectly—it's in the mindset shift toward understanding and modeling the business domain properly.

Now that I've gone through this deep dive, I'm excited to apply these concepts more broadly and see how they improve the maintainability and clarity of complex applications.